The present invention relates to a method of manufacturing soybean curd employing microwave processing.
Traditionally, soybean curd has been made by adding calcium sulfate, calcium chloride, or agents similar to these inorganic chemicals, to hot soybeam milk. These coagulating agents have been found to be very effective. Coagulation is followed by pressing to expel the whey from the curd and to shape-form the curd. This traditional method of production is entirely batch oriented.
A new acidic type coagulating agent, glucono-delta-lactone, (hereafter "GDL") has been developed in the 1960's. This agent produces an even, fine-grained soybean curd. Using GDL as the coagulation agent, the accepted practice of manufacturing soybean curd is as follows: After the soybean milk is boiled and filtered, it is cooled to room temperature. GDL is added to the soybean milk and thoroughly mixed therein. Packaging containers are filled with this soybean milk-coagulant mixture. These containers are immersed in a constant temperature hot water bath for a long period of time until the curd is formed. This process has been developed to a state of art by which continuous production of soybean curd is possible. Prior art relating to the use of GDL in the manufacture of soybean curd includes the following U.S. Pat. Nos. 4,000,326; 4,140,811; 4,514,433; 4,537,789; 4,585,665; 4,636,398; 4,789,556; 4,826,701; and 4,828,869.
GDL is derived from glucose and forms white, odorless crystals. It is used extensively in the food industry as an acidulant. In water, it is hydrolyzed and decomposes to give gluconic acid, which is the actual active agent as acidulant. Likewise, for the coagulation of soybean curd, gluconic acid is the active coagulation agent.
GDL is stable in powder form. It is freely soluble in water. When added to water, it slowly hydrolyzes to form an equilibrium mixture of gluconic acid and its delta- and gamma-lactones, as shown in the following equation. ##STR1##
At room temperature, complete hydrolysis occurs in about three hours. FIG. 1 shows the extent of acid formation with increase in elapsed time of buffered aqueous GDL solutions at various concentration levels at 20.degree. C. If curves No. 1 and No. 2 of FIG. 1 are compared, it is noted that for solutions of same concentration, the initial hydrolysis rate is much higher for a more neutral solution than for a more acidic solution. Gluconic acid being the coagulant, a higher release rate of gluconic acid will give the coagulant a higher concentration immediately and, after any total elapsed time from the initial introduction of the gluconic acid, a longer period of higher concentration to diffuse throughout the soybean milk. It can also be concluded that in a non-buffered solution as hydrolysis proceeds the amount of gluconic acid that is formed will actually depress the subsequent rate of hydrolysis of GDL. But a depletion of gluconic acid, such as occurs when gluconic acid is removed from the soybean milk as it coagulates the soybean milk into soybean curd, will induce more hydrolysis of the lactones.
Another notable property of the rate of hydrolysis of GDL can be studied by referring to FIG. 2 which is a plot of degree of hydrolysis, represented in terms of pH value of the solution vs. elapsed time for a 1% GDL aqueous solution at temperatures of 5.degree., 20.degree., 50.degree., and 90.degree. C. These curves all show inverted exponential characteristics, i.e., the rate increases rapidly during initial stage, reaches a maximum, and decreases to a minimum value, with this feature becoming more pronounced at higher temperatures of 50.degree. C. and 90.degree. C. It can be seen that at 90.degree. C. an equilibrium pH value of 2.5-2.6 is reached after an elapsed time of only about 5 minutes. It is thus most advantageous to operate the coagulation at a temperature of about 90.degree. C. to achieve maximum rate of hydrolysis of GDL.
The industrial practice in Japan for the production of soybean curd, or, by its Japanese name, "tofu," is to operate the coagulation step at 80.degree.-90.degree. C. by means of a hot water bath and with the immersion time being 40-50 minutes to give good coagulation results. However, the machinery for a hot water bath for continuous production is complex and costly to build. Also, the processing efficiency is lowered because of the time needed to conduct heat from the hot water bath into the contents of containers of soybean milk-coagulant liquid mixture to raise the temperature of the liquid.